In semiconductor integrated circuit (IC) fabrication, metal lines are deposited to interconnect IC components and to connect IC components to pads. The metal lines are formed by physical deposition (such as by sputtering) of a layer of metal (such as aluminum). Photoresist is applied to the metal layer to define a pattern for forming lines that interconnect the desired components of the IC. Referring to FIG. 1, metal lines 10 are etched according to the pattern defined by the photoresist 14. One common apparatus used for such etching is the Applied Materials MxP Centura.
The Mxp Centura has four chambers: two chambers are etch chambers and two chambers are strip chambers which are called ASP chambers. A typical metal etching process is performed in the etch chamber at about 80 degrees centigrade. During a metal line etching process, the metal lines are etched using a chloride plasma, which will result in chloride formed on the surface of the metal lines. Furthermore, polymers 12 are also usually formed on the sidewalls of metal lines 10.
Referring to FIG. 3, after the metal lines 10 are etched in the etch chamber at etch step 30, the wafer is transferred from the etch chamber to the strip chamber (ASP chamber). The photoresist is then stripped in the strip chamber at a strip step 31, usually with a dry etching process at about 250 degrees centigrade. Afterward, the wafer is taken out of the strip chamber to cool the wafer. To remove the photoresist residue and polymer, two polymer strip steps (PRS) 32, 34 and one "Mattson" step 33 between the two polymer strip steps are then performed.
Referring to FIG. 4, a detailed flow diagram of strip step 31 of FIG. 3 is shown. A stabilization step 40 is performed, and gases, such as N.sub.2, H.sub.2 O and O.sub.2, are inserted into the strip chamber to stabilize the response condition. In the strip chamber, at a temperature of about 250 degrees centigrade, an etching chloride process 41 and a stripping photoresist process 42 (together referred to as an strip cycle 43) are repeatedly performed several times to remove the chloride and the photoresist. Then, a pump is applied to take the response gases out of the strip chamber in a pumping step 44.
A typical removing chloride recipe is described as follows: 500 sccm H.sub.2 O/1400 watts/2 Torrs/10 seconds (a gas flow of H.sub.2 O at about 500 sccm; an energy of etching at about 1400 watts; a gas pressure of the chamber at about 2 Torrs; an removing time of about 10 seconds). A typical strip photoresist recipe is described as follows: 300 sccm H.sub.2 O/3500 sccm O.sub.2 /200 sccm N.sub.2 /1400 watts/2 Torrs/20 seconds (a gas flow of H.sub.2 O at about 300 sccm; a gas flow of O.sub.2 at about 3500 sccm; a gas flow of N.sub.2 at about 200 sccm; an energy of etching at about 1400 watts; a gas pressure of chamber at about 2 Torrs; an removing time of about 20 seconds). When the wafer is transferred form the etch chamber (80 degrees centigrade) to the strip chamber (250 degrees centigrade), the temperature difference between the wafer and the strip chamber is large. Indeed, there is a large temperature differential of about 170 degrees centigrade. Therefore, the time for the temperature to balance between the wafer and strip chamber is relatively long. Further, the strip cycle may be repeated a number of times, and may be repeated up to seven times.
Even with this cleaning technique, photoresist residue is sometimes observed on the metal lines. With the more densely packed metal layout for sub-micron devices, the impact of photoresist residue will be more noticeable. The photoresist residue 16 on densely packed metal lines 10 may remain on the metal lines 10, regardless of the number of times the stripping step is performed, as shown in FIG. 2. Further, the photoresist residue is difficult to detect during after-etch inspection (AEI). Also, in processing after the strip cycles, removal of the polymer 12 from the metal lines 10, as shown in FIG. 2, typically includes two stripping polymer steps (PRS) 32, 34. Additionally, it is necessary for a stripping photoresist residue step (Mattson) 33 to be added between the two PRS steps to remove photoresist residue remaining on the metal lines. Thus, there is an unmet need in the art for a method for preventing photoresist residue from forming on metal lines.